Add README.md to 'calculator' sample (#1115)

samples/calculator/README.md

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+# iOS calculator sample
+
+This example shows how to use Kotlin library compiled to framework from
+an existing iOS project (e.g. written in Swift or Objective-C).
+
+To build and run the sample do the following:
+
+1.  Open `samples/calculator/calculator.xcodeproj` with Xcode.
+2.  Open the project's target through project navigator, go to tab 'General'.
+    In 'Identity' section change the bundle ID to the unique string in
+    reverse-DNS format. Then select the team in 'Signing' section.
+    
+    See the
+    [Xcode documentation](https://developer.apple.com/library/content/documentation/IDEs/Conceptual/AppDistributionGuide/ConfiguringYourApp/ConfiguringYourApp.html#//apple_ref/doc/uid/TP40012582-CH28-SW2)
+    for more info.
+3.  Now build and run the application on a connected iPhone with Xcode.
+
+The sample consists of:
+
+1.  Xcode iOS application project, written in Swift. It uses Kotlin library to
+    parse simple arithmetic expressions.
+2.  Kotlin library source code and build script. It is built into Objective-C
+    framework by invoking Gradle from custom "Run Script" build phase, and this
+    framework is imported into the Xcode project.
+
+    The library can also be compiled to a `.jar` by Kotlin/JVM compiler with
+    Gradle.